ASP.NET 生成缩略图(防失真)。

BuildSourImage()，避免了使用传统的 SaveAs()方法保存文件。

BuildThumbImage()方法，是生成缩略图的，这个生成缩略图的步骤是，首先读取原图。根据原图的大小，按照比例缩放原图。像下面的代码： 

            //问题1              //这个判断就是判断，哪一个除数大（即高度大），那长度不变，高度乘以比例。反之，类推。              if (((decimal)width) / height <= ((decimal)intThumbWidth) / intThumbHeight)            {                smallWidth = intThumbWidth;                smallHeight = intThumbWidth * height / width;                //问题2 <br>                  //其实可以写成下面这样子，不知道对不对？我的理解就是，拿缩略图的和原图的宽的比例，然后乘以高度。就是两者比例下的高度了。                 //smallHeight = intThumbWidth / width * height;               }            else {                                       smallWidth = intThumbHeight / height * width;                    smallHeight = intThumbHeight;             } 

全部代码如下
        

        protected void Button1_Click(object sender, EventArgs e)        {             UpLoadImage(File1, "~/UPLOAD/", "X.JPG", 20, 15);        }        /// <summary>           /// asp.net上传图片并生成缩略图           /// </summary>           /// <param name="upImage">HtmlInputFile控件</param>           /// <param name="sSavePath">保存的路径,些为相对服务器路径的下的文件夹</param>           /// <param name="sThumbExtension">缩略图的thumb</param>           /// <param name="intThumbWidth">生成缩略图的宽度</param>           /// <param name="intThumbHeight">生成缩略图的高度</param>           /// <returns>缩略图名称</returns>           public string UpLoadImage(HtmlInputFile upImage, string sSavePath, string sThumbExtension, int intThumbWidth, int intThumbHeight)        {            string sThumbFile = "";            string sFilename = "";            if (upImage.PostedFile != null)            {                sFilename = BuildSourImage(upImage, sSavePath);//上传原图                  sThumbFile = BuildThumbImage(sSavePath + sFilename, sFilename, sSavePath + "small/", 300, 200); //上传缩略图                  return sThumbFile;            }            return "没有选择图片";        }        /// <summary>          /// 上传大图          /// </summary>          /// <param name="upImage"></param>          /// <param name="sSavePath"></param>          /// <returns></returns>          public string BuildSourImage(HtmlInputFile upImage, string sSavePath)        {            string sFilename = "";            HttpPostedFile myFile = upImage.PostedFile;            int nFileLen = myFile.ContentLength;            if (nFileLen == 0)                return "没有选择上传图片";            //获取upImage选择文件的扩展名               string extendName = System.IO.Path.GetExtension(myFile.FileName).ToLower();            //判断是否为图片格式               if (extendName != ".jpg" && extendName != ".jpge" && extendName != ".gif" && extendName != ".bmp" && extendName != ".png")                return "图片格式不正确";            byte[] myData = new Byte[nFileLen];            myFile.InputStream.Read(myData, 0, nFileLen);            sFilename = DateTime.Now.ToString("yyyyMMddHHmmssfff") + extendName;            //检查当前文件夹下是否有同名图片,有则在文件名+1               System.IO.FileStream newFile                = new System.IO.FileStream(System.Web.HttpContext.Current.Server.MapPath(sSavePath) + sFilename,                System.IO.FileMode.Create, System.IO.FileAccess.Write);            newFile.Write(myData, 0, myData.Length);            newFile.Close();            return sFilename;        }        /// <summary>          /// 生成缩略图          /// </summary>          /// <param name="sSourceFile">原图地址</param>          /// <param name="thumbName">缩略图名称</param>          /// <param name="sSavePath">保存地址</param>          /// <param name="intThumbWidth">缩略图宽度</param>          /// <param name="intThumbHeight">缩略图高度</param>          /// <returns></returns>          public string BuildThumbImage(string sSourceFile, string thumbName, string sSavePath, int intThumbWidth, int intThumbHeight)        {            try            {                //原图加载                   using (System.Drawing.Image sourceImage = System.Drawing.Image.FromFile(System.Web.HttpContext.Current.Server.MapPath(sSourceFile)))                {                    //原图宽度和高度                       int width = sourceImage.Width;                    int height = sourceImage.Height;                    int smallWidth;                    int smallHeight;                    //获取第一张绘制图的大小,(比较 原图的宽/缩略图的宽 和 原图的高/缩略图的高)                       if (((decimal)width) / height <= ((decimal)intThumbWidth) / intThumbHeight)                    {                        smallWidth = intThumbWidth;                        smallHeight = intThumbWidth * height / width;                    }                    else                    {                        smallWidth = intThumbHeight * width / height;                        smallHeight = intThumbHeight;                    }                    //缩略图保存的绝对路径                       string smallImagePath = System.Web.HttpContext.Current.Server.MapPath(sSavePath) + thumbName;                    //新建一个图板,以最小等比例压缩大小绘制原图                       using (System.Drawing.Image bitmap = new System.Drawing.Bitmap(smallWidth, smallHeight))                    {                        //绘制中间图                          using (System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap))                        {                            //高清,平滑                               g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;                            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;                            g.Clear(Color.Black);                            g.DrawImage(                            sourceImage,                            new System.Drawing.Rectangle(0, 0, smallWidth, smallHeight),                            new System.Drawing.Rectangle(0, 0, width, height),                            System.Drawing.GraphicsUnit.Pixel                            );                        }                        //新建一个图板,以缩略图大小绘制中间图                           using (System.Drawing.Image bitmap1 = new System.Drawing.Bitmap(intThumbWidth, intThumbHeight))                        {                            //绘制缩略图                               using (System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap1))                            {                                //高清,平滑                                   g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;                                g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;                                g.Clear(Color.Black);                                int lwidth = (smallWidth - intThumbWidth) / 2;                                int bheight = (smallHeight - intThumbHeight) / 2;                                g.DrawImage(bitmap,                                    new Rectangle(0, 0, intThumbWidth, intThumbHeight),                                    lwidth, bheight, intThumbWidth, intThumbHeight,                                    GraphicsUnit.Pixel);                                g.Dispose();                                bitmap1.Save(smallImagePath, System.Drawing.Imaging.ImageFormat.Jpeg);                            }                        }                    }                }            }            catch            {                //出错则删除                   System.IO.File.Delete(System.Web.HttpContext.Current.Server.MapPath(sSavePath + thumbName));                return "图片格式不正确";            }            return thumbName;        }